1. Field of the Invention
This invention relates to a calcium ion-selective electrode and to a calcium ion-selective membrane for use therein.
2. Description of the Prior Art
Extensive research has been undertaken in the area of calcium ion-selective electrodes as evidenced by the extensive patent (5-10) and technical (11-30) literature in this area and the development and state of the art with respect to these electrodes has been documented (1-4).
More specifically, U.S. Pat. No. 3,729,401 (8) discloses the use of the diester of t-octylphenyl phosphoric acid as the calcium ion selector dissolved in an organic liquid such as di-n-octylphenylphosphonate. U.S. Pat. No. 3,932,233 (10) is directed to the calcium salts of di-(aryl)phosphates and di(n-alkylaryl) phosphates in admixture with their respective acid phosphates and combined with polyvinyl chloride and di-n-octyl phenylphosphonate. Griffiths et al. (13) formed a solid membrane system using polyvinyl chloride. Griffiths et al. (13) found that there was an optimum mixture in formulating the calcium membrane when monocalcium dihydrogen tetra(didecylphosphate) was used in a weight ratio of 1:10 with di-n-octyl phenylphosphonate plasticizer. This latter mixture was used in 71.2% concentrations by weight with 28.8% polyvinyl chloride homopolymer added. The calcium acid tetra salt of the diester phosphate was made up by mixing equal portions of the diester acid phosphate with monocalcium di(didecylphosphate). The work of Griffiths et al. (13) was classic in calcium ion electrode technology and has been copied by many workers in the art. Thus, it is considered necessary by those skilled in the art to incorporate the ion sensing phosphate ligand as a calcium acid salt form into the plasticized membrane. Hence, all purifications of the diester phosphates have involved the isolation of both the diester acid phosphate and the calcium salt diester phosphate and then combining the two equivalent ingredients into the phosphonate plasticizer in the 1:10 ratio.
Recently, Moody et al. (30) have disclosed that one can obtain satisfactory results using just the calcium salt of bis[di(p-1,1,2,2-tetramethylbutylphenyl) phosphate] in conjunction with a plasticizer in a ratio of up to 1 part calcium salt to 10 parts plasticizer.
However, serious shortcomings exist with the prior art calcium electrodes. These shortcomings include (1) a relatively slow response of the calcium electrode; (2) a relatively poor selectivity to calcium in the lower ion concentration ranges; (3) a relatively poor use life; and (4) a relatively difficult manufacturing process entailed in the manufacture thereof.
It would therefore be very advantageous to have a calcium electrode and membrane for use therein which overcomes these problems present in the prior art.